Okay, so the Durability feat states that lethal damage equal to or below the PC's damage save bonus becomes stun damage, so the character can only be knocked unconscious unless the damage exceeds the PC's damage save. However, when a character is unconscious they turn stun damage into lethal damage, but how does the Durability feat affect a character who is unconscious?

I can think of four different ways this could work, and I'd like to know which one is correct.

1. Durability equals Invulnerability.
The character continues to take stun damage, even while unconscious, unless the damage is lethal and exceeds his damage save.

2. Lethal damage is Lethal again because he's unconscious.
Because the character is unconscious, the affects of Durability only protect him from from stun damage becoming lethal damage, but lethal damage remains lethal, regardless of what it is relative to his Damage save bonus.

3. All damage must exceed his Damage save bonus.
The damage that affects the character, whether originally stun or lethal, must exceed his damage save bonus to convert to lethal damage, otherwise it is considered stun damage.

4. Sorry dude, you're boned.
All damage, stun or lethal, is considered lethal damage to the character because he is unconscious, as is he didn't have the feat while unconscious.

Which one of these answers is correct?
Plus, I'll give an example and see if this helps to answer the question.

Hypothetical: A player, Johnny Paranoid, decides to make a character that is REALLY tough to defeat. He gives the character 20 constitution, 10 ranks in super-constitution, and the Toughness and Durability feats. This means that any lethal damage under 18 is converted to stun damage instead.
After two game sessions, the game master gets tired of not being able to hurt Johnny with his villains, and decides to create a villain named Lady Lightning, who has energy blast +15. After a few very unfortunate rolls by Johnny, he is knocked unconscious. Lady Lightning stands over Johnny and says "Game over." and zaps him with a blast of lightning.
What happens?